In electromagnetic echo-detection systems, digital filters are used in the video circuits of components designed to eliminate signals from fixed-target echoes; these components are known as "moving-target indicators" abbreviated M.T.I.
The filters referred to operate by effecting a comparison of the signals obtained at time t with those obtained under the same conditions at time t+.DELTA.t. However, the structure of such filters rapidly becomes complex when multi-stage filtering is contemplated. These filters may break down increasingly with the increasing complexity of their structure and, under operating conditions, such failures are not easily detectable. Furthermore, as far as we are aware, such filters are generally not checked under operating conditions but are tested before being used and, if need be, may be subsequently retested outside the system which uses them.